Electroosmosis is an electrokinetic phenomenon that occurs when an electrolyte fluid interacts with solid surfaces causing a charged layer to form at the interface between the solid and the liquid. Immobilized electric charges develop at the surface of the solid surface in contact with the electrolyte fluid due to electro-chemical phenomena. The surface charge leads to the formation of an electric double layer (“EDL”) by influencing the distribution of counter-ions and co-ions in the electrolyte fluid. In a diffuse layer of the EDL, the counter-ions predominate over the co-ions to neutralize the surface charge. As such, the local net charge density is not zero. A Columbic force is exerted on the ions within the EDL when an electric field is applied tangentially along the charged surface. Consequently, an electroosmotic flow (EOF) results whereby the migration of mobile ions will carry the adjacent and bulk liquid phase by viscosity.
The build-up of pressure as a result of electroosmosis facilitates the transport and manipulation of liquids in microfluidic devices for biomedical applications. These principles have been applied in the operation of many electroosmotic pumps. Such electroosmotic pumps work without movable mechanical parts, consequently improving durability and minimizing difficulties in production. Such electroosmotic pumps are essential for biochemical analyses as they enable the pumping of liquids over a wide range of fluid conductivities.
Given that electroosmosis is essentially a surface dominated phenomenon, the use of a porous structure with a high surface area-to-volume ratio can enhance the pressure-building capacity. Paul et al. [1998 Electrokinetic generation of high pressures using porous microstructures in: Proceedings of the Micro Total Analysis Systems '98 Workshop, Banff, Canada] proposed a method to generate high pressure using DC electroosmosis through a microchannel packed with microparticles. The pressure of 10 atm at 1.5 kV applied voltage has been achieved using fused-silica capillaries packed with charged 1.5 μm silica beads. S. Zeng et al, [Fabrication and Characterization of Electroosmotic Micropupms, Sensors and Actuators B 2001, 79, 107-114] fabricated an electroosmotic pump that can generate maximum pressures in excess of 20 atm or maximum flow rates of 3.6 μl/min by applying a 2 kV electric voltage over 5.4 cm long, 500-700 μm in diameter fused-silica capillaries packed with 3.5 μm silica particles. S. Yao et al, [A Large Flowrate Electroosmotic Pump with Micro Pores, Proceedings of IMECE, ASME, 2001, New York, N.Y.] developed an electroosmotic pump for a large flowrate with micro pores which can generate a maximum flowrate of 7 ml/min and a maximum pressure of 2.5 atm for 200V applied potential. In a recent development, L. Chen et al, [Generating High-Pressure Sub-Microliter Flow Rate in Packed Microchannel by Electroosmotic Force: Potential Application in Microfluidic Systems, Sensors and Actuators B 2003 88 260-265] developed a pump made of microchannels packed with porous fine dielectric material, which can generate a maximum pressure of 15 MPa.
The aforementioned sampling of documents show that the use of electroosmotic principles are commonly employed in micro-fluid pumping. Thus far, there has been no disclosure of the application of electroosmotic principles for actuation.